In mobile communication technologies like, e.g., UMTS (Universal Mobile Telecommunication System) or GSM (Global System for Mobile telecommunication), base stations serve a limited number of mobile users according to the current location of the users. As long as a user is in a base station cell area, he can obtain mobile services from that base station. Due to the inherent mobility in the system, available capacity in one cell might not be used whereas in another cell there can be too many users in the cell area at the same time. Cell areas of usually different sizes and a varying user density increase this inbalance, thus wasting nominally available and expensive network capacity.
In UMTS networks this problem is emphasized in downlink direction, i.e. from base station to mobile station. This is because most probably the downlink will be the capacity limiting direction due to the downlink intensive services such as for example web-browsing or streaming video
Despite thorough network planning, hard or soft limits of a mobile phone network can refuse additional mobile phone users to get service in one cell even if some spare capacity is left in other cells, i.e. at other locations of the network. Thus, service can not be provided even if the network as a whole could support more services.
This is in particular a problem in case high capacity is needed temporarily, as, for example, during large sport events or open air festivals which occur only a few times in a year. A solution in which additional base stations are provided involves high costs and is therefore not acceptable.